LT1952/LT1952-1
Single Switch Synchronous
Forward Controller
FEATURES
DESCRIPTION
TheLT®1952/LT1952-1arecurrentmodePWMcontrollers
optimizedtocontroltheforwardconvertertopology,using
one primary MOSFET. The LT1952/LT1952-1 provide
synchronous rectifier control, resulting in extremely
high efficiency. A programmable Volt-Second clamp
provides a safeguard for transformer reset that prevents
saturation. This allows a single MOSFET on the primary
side to reliably run at greater than 50% duty cycle for high
MOSFET, transformer and rectifier utilization. The devices
include soft-start for controlled exit from shutdown and
undervoltage lockout. A precision 107mV current limit
threshold, independent of duty cycle, combines with soft-
starttoprovidehiccupshort-circuitprotection.TheLT1952
is optimized for micropower bootstrap start-up from high
input voltages. The LT1952-1 allows start-up from lower
input voltages. Programmable slope compensation and
leadingedgeblankingallowoptimizationofloopbandwidth
with a wide range of inductors and MOSFETs. Each device
can be programmed over a 100kHz to 500kHz frequency
range and the part can be synchronized to an external
clock. The error amplifier is a true op amp, allowing a wide
range of compensation networks. The LT1952/LT1952-1
are available in a small 16-pin SSOP package.
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Synchronous Rectifier Control for High Efficiency
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Programmable Volt-Second Clamp
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Output Power Levels from 25W to 500W
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Low Current Start-Up
(LT1952: 460μA; V On/Off = 14.25V/8.75V)
IN
(LT1952-1: 400μA; V On/Off = 7.75V/6.5V)
IN
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True PWM Soft-Start
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Low Stress Short-Circuit Protection
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Precision 107mV Current Limit Threshold
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Adjustable Delay for Synchronous Timing
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Accurate Shutdown Threshold with Programmable
Hysteresis
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Programmable Slope Compensation
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Programmable Leading Edge Blanking
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Programmable Frequency (100kHz to 500kHz)
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Synchronizable to an External Clock up to 1.5 • f
OSC
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Internal 1.23V Reference
2.5V External Reference
Current Mode Control
Small 16-Pin SSOP Package
APPLICATIONS
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Telecommunications Power Supplies
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Industrial and Distributed Power
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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Isolated and Non Isolated DC/DC Converters
TYPICAL APPLICATION
36V to 72V Input, 12V at 20A Semi-Regulated Bus Converter
12V Bus Converter
OUT vs VIN
L1
V
T1
V
IN
PA1494.242
V
12V
20A
PA0905
OUT
SUPPLY FROM BIAS
WINDING OF T1
16
14
12
10
8
10μF
47μF
16V
X5R
s2
V
V
IN
REF
52.3k
100k
Si7370
s2
PH4840
s2
COMP
Si7450
SS_MAXDC
OUT
OC
V
IN
340k
LT1952/
LT1952-1
I
0.005Ω
T2
SENSE
SD_V
FB
SEC
LTC3900
FG
CG
13k
SYNC
GND
SOUT
SYNC
560Ω
R
220pF
OSC
36
48
54
(V)
60
66
72
42
V
PGND BLANK DELAY
40k 40k
IN
0.1μF
0.1μF
1952 TA01b
178k
1952 TA01
19521fd
1